The present invention relates to a brake system with an adjustably variable front/rear axle braking force distribution for a vehicle designed for high speeds, in particular a racing car, in which downward aerodynamic forces on the vehicle cause an increase in axle loads as vehicle speed increases, wherein the increase in axle load is greater at the rear axle than at the front axle, comprising front wheel brakes, rear wheel brakes, a brake unit actuatable by a brake pedal and configured to generate at least a brake pressure in the front wheel brakes and operatively associated with the front wheel brakes and the rear wheel brakes that in the event of a specified defined limiting value of the actuation force exerted by a driver of the high speed vehicle, the vehicle has a magnitude of deceleration corresponding to complete utilization of frictional force on the braked vehicle wheels.
At the high speeds of 350 km/h and more which are attained by racing cars on high-speed tracks, and given the aerodynamic shapes of the bodies for such vehicles which usually have a rear-axle drive, drastic increases in axle load occur at both the front axle and the rear axle because of the aerodynamic downward forces which increase quadratically with increasing vehicle speed. In magnitude, these increases in axle load can correspond to a multiple of the vehicle weight, for example twice or three times the stationary vehicle weight. For the designs possible, it is found that with increasing vehicle speed v, the aerodynamic downward forces effective at the rear axle increase more strongly than the aerodynamic downward forces effective at the front axle. This, however, leads to the fact that a brake system, such as one designed for a fixed matching of the braking force distribution, can only be designed in an optimum manner for a narrow speed range and, outside this speed range, is associated with either the danger of unstable dynamic braking behaviour of the vehicle or the disadvantage of a relatively low braking deceleration for a specified actuating force.
This disadvantage can indeed be ameliorated by an anti-lock brake system (ABS) which operates with a speed-dependent change to the response threshold both at the front axle and at the rear axle as discussed in DE 39 35 834 C2. Because, however, the brake system must be basically matched for stable braking behaviour without the influence of the aerodynamic forces, so that the brake coefficient B*.sub.H of the rear axle must of necessity be kept relatively low, a disproportionately large increase in pressure is necessary at the front axle when the anti-lock brake system responds in the high speed range in order to enforce a response of the control system at the rear wheel brakes also. However, the expenditure of force necessary for such an increase in pressure, on the one hand, and the time that elapses before the build-up of pressure is achieved limit the effectiveness of an anti-lock braking system for racing duties and lead, as a result, to time delays in the build-up of brake pressure which lead, as a result, to unacceptable losses of time when racing.
On the other hand, twin main cylinders are also used in racing cars in which "single circuit" main cylinders associated with the front wheel brakes and the rear wheel brakes are actuated by a rocker with a variable point of action of the force, the most favorable positional setting of the point of action of the force being different for each speed. In combination with twin main cylinders, automatically operating adjustment appliances for the point of action of the force on the rocker are also known which would, in principle, also permit a speed-dependent variation in the braking force distribution. Such adjustment appliances, however, have the disadvantage that very high forces act in braking operation on the linkage piece marking the point of action so that the adjustability is necessarily associated with hysteresis and substantial adjustment times have to be accepted, with the result that an exact setting of the optimum braking force distribution is practically impossible. For this reason, in fact, automatically operating adjusting devices of the type described have not previously been employed in racing and only those which can be set "by hand" to the statistically most important situation have been used.
An object of the present invention is, therefore, to improve a brake system such that it permits rapid setting, to meet the requirements, of an optimized braking force distribution with, at the same time, a simple functionally reliable construction suitable for use in a racing vehicle.
This object has been achieved in accordance with the present invention in that in addition to a brake unit, a further brake pressure source is provided whose outlet pressure is electronically controllable so that over the complete speed range which has to be considered, there is a distribution corresponding to the ideal or almost to the ideal braking force distribution of the braking forces which can be exerted by the front wheel brakes and the rear wheel brakes and, referred to the actuating force which the driver has to apply during a braking operation, the maximum possible vehicle deceleration. The algorithm employed, according to the present invention, for controlling the outlet pressure of the additional brake pressure source is based on the condition of equality of the wheel lock limits at the front wheels and the rear wheels and corresponds to an approximate numerical evaluation of this relationship. This algorithm can be evaluated very rapidly electronically and a rapid follow-up of the rear axle brake pressure in relation to the front axle brake pressure, taking account of the respective vehicle speed, is therefore also possible. The connection of the outlet pressure of the brake pressure source into the rear wheel brake caliper piston pairs to be acted upon by this outlet pressure is possible with simple structural features without a complicated brake unit being necessary for this purpose. This applies particularly where the rear wheel brakes are acted upon exclusively by the outlet pressure of the controllable pressure source, in which case it is only the brake pressure to be connected to the front wheel brakes which must be generated by the brake unit, which can therefore be configured as a simple "single circuit" main cylinder and also, if appropriate, as a tandem main cylinder to each of whose outlet pressure spaces is connected one of the front wheel brakes.
In an embodiment of the brake system of the present invention in which the rear wheel brakes have four-piston calipers with the outlet pressure of the controllable brake pressure source, respectively connected to only one of these piston pairs, the additional brake pressure source can be configured for a lower outlet pressure level, and increased safety relative to a failure of the additional pressure source is also provided because the rear wheel brake caliper piston pair, which is acted on by brake pressure by the brake unit, remains functional even in the case of a failure of the additional brake pressure source.
With the aid of a tandem-main cylinder modified to have a control pressure space which, when it is acted on by the outlet pressure of the controllable pressure source, permits the attainment of the development of brake pressure in the rear wheel brakes relative to the brake pressure in the front wheel brakes corresponding to the optimized braking force distribution an advantageous simple conduit routing for the brake conduits necessary for the front wheel brakes and the rear wheel brakes is achieved and permits an arbitrary configuration of the brake calipers.
In combination with this, a simple configuration of the tandem main cylinder with a control pressure space is provided, in one embodiment of which a reaction of the control pressure employed for pressure modulation of the rear axle brake pressure on the front axle brake circuit is avoided.
The brake system configuration provided in accordance with an embodiment of the present invention advantageously ensures that the vehicle can be braked in a dynamically stable manner even in the case of a failure of the additional brake pressure or control pressure source.
Further features of the present invention provide embodiments of the controllable pressure source which permit, by simple technical control techniques, sensitive continuous or quasi-continuous setting or follow-up of the brake pressure connected into the rear wheel brakes.
Knowledge of the braking force developed by the front wheel brakes is necessary for the appropriate control of the braking force to be developed by the rear wheel brakes. This knowledge can be gained in a simple manner by a pressure pick-up associated with the front axle brake circuit or a position pick-up recording the position of the brake pedal. The pressure pick-up has the advantage that the measured parameters of interest can be more accurately determined but it requires an additional seal location in the hydraulic conduit system whereas the position pick-up can be arranged "outside" the conduit system and therefore provides the advantage of less susceptibility to fault of the brake system.
A pressure pick-up is present for recording the actual value of the pressure connected into the rear wheel brakes, with the pressure pick-up being expediently connected directly at the pressure outlet of the controllable pressure source. In such embodiment, the rear axle braking force can follow up the front axle braking force in the relationship provided so as to provide a follow-up control with particularly good maintenance of the most favorable braking force distribution ratio.
Knowledge of the magnitude of the friction coefficient .mu..sub.B currently effective between the roadway and the braked vehicle wheels is desirable for optimized braking force distribution control. This knowledge can be gained in a simple manner from the dynamic acceleration behavior of the vehicle and/or from the deceleration behavior of the vehicle when the anti-lock control is effective during a braking operation on all the vehicle wheels. For these two ways of recording the friction coefficient, it is advantageous for the vehicle to be equipped with a longitudinal acceleration sensor which generates electrical output signals characteristic of the accelerations and/or decelerations occurring on the vehicle, which output signals can be employed for calculating the required value of the currently most favorable braking force distribution.